This invention relates to magnetic elements and, in particular, to semi-hard magnetic elements and methods of making same.
As used herein, the term semi-hard magnetic element means a magnetic element having semi-hard magnetic properties which are defined herein as a coercivity in the range of about 10-500 Oersted (Oe) and a remanence, after removal of a DC magnetization field which magnetizes the element substantially to saturation, of about 6 kilogauss (kG) or higher. Semi-hard magnetic elements having these semi-hard magnetic properties have been used in a number of applications. In one particular application, the elements serve as control elements for markers in a magnetic electronic article surveillance (EAS) system. A magnetic marker of this type is disclosed, for example, in U.S. Pat. No. 4,510,489.
In the marker of the '489 patent, a semi-hard magnetic element is placed adjacent to a magnetostrictive amorphous element. By magnetizing the semi-hard magnetic element substantially to saturation, the resultant remanence magnetic induction of the magnetic element arms or activates the magnetostrictive element so that it can magnetically resonant or vibrate at a predetermined frequency in response to an interrogating magnetic field.
This mechanical vibration results in the magnetostrictive element generating a magnetic field at the predetermined frequency. The generated field can then be sensed to detect the presence of the marker. By demagnetizing the semi-hard magnetic element, the magnetostrictive element is disarmed or deactivated so that it can no longer mechanically resonate at the predetermined frequency in response to the applied field. This type of marker is sometimes referred to as a "magnetomechanical" marker, and the corresponding EAS system is referred to as a magnetomechanical EAS system.
A technique for producing low-cost semi-hard magnetic elements usable as control elements in magnetomechanical markers was disclosed in U.S. Pat. No. 5,351,033, which is commonly assigned with the present application. According to the disclosure of the '033 patent, amorphous metalloid materials, such as Metglas.RTM. 2605TCA and 2605S2, which have soft magnetic properties as cast, are processed so that the materials develop semi-hard magnetic properties. The process disclosed in the '033 patent includes cutting the as-cast amorphous alloy ribbons into discrete strips and then annealing the strips so that at least a part of the bulk of the strips is crystallized.
When amorphous alloy strips are treated in accordance with the teachings of the '033 patent, there is a reduction in volume that accompanies the transition from an amorphous phase to a crystalline phase. In general, the reduction in volume is anisotropic, and as a consequence, the alloy strips are subjected to warping which leaves the strips in a deformed or "rippled" condition.
There are a number of disadvantages which result from the rippled shape of the processed strips. Among these are difficulties in handling the strips and packaging the strips to form the EAS markers. Further, if the magnetic elements are used as control elements in their rippled condition, there are variations from element to element in terms of the effective bias field provided by the control element, which results in variations in the resonant frequency of the assembled markers. This can adversely affect the performance and reliability of the markers.